1. Field of the Invention
The present invention relates to a circuit for driving an inductive load, and in particular, to the circuit in which a flywheel diode is used.
2. Description of the Related Art
In general, inductive loads, such as electric motors, are driven by making use of inverters that require switching elements. Power MOSFETs are frequently used as the switching elements. In such a case, it is frequent that, in addition to the function as the mere switching element, the power MOSFET is composed to have the function as a flywheel diode by making use of an internal body diode embedded within the element.
However, the body diode embedded in the power MOSFET has generally a poor recovery characteristic (i.e., poor reverse recovery characteristic), thus giving rise to higher recovery surge voltage and an oscillation phenomenon called ringing. In some cases, such undesirable phenomena will lead to element breakage and noise generation. Accordingly, the poor recovery characteristic of the body diode becomes an obstacle to a satisfactory performance of a product in which the power MOSFET is incorporated.
The present invention has been made to overcome the above problem. An object of the present invention is to provide a flywheel diode whose recovery characteristic (i.e., reverse recovery characteristic) is softened (or smoothed) with ease so that recovery surges are suppressed in a well controlled manner.
In order to achieve the foregoing object, as one aspect of the present invention, there is provided a reflux closed circuit connected to an inductive load, the circuit comprising: a flywheel diode inserted in the reflux closed circuit; a MOS transistor connected in parallel to the flywheel diode; a capacitor of which both ends are connected to a gate and a drain of the MOS transistor; a resistor of which both ends are connected to the gate and a source of the MOS transistor; and means for controlling a gate-to-source voltage of the MOS transistor by causing both the capacitor and resistor to raise the gate-to-source voltage up to a value over a threshold voltage given the MOS transistor for a predetermined period of time during a recovery operation of the flywheel diode.
Accordingly, when the flywheel diode is in its recovery (i.e., reverse recovery) operation, both of the capacitor and the resistor allows a gate-to-source voltage of the MOS transistor to be pulled up over the predetermined threshold for a specified period of time. Thus, the recovery characteristic of the flywheel diode can be softened (or smoothed), thus recovery surges being suppressed.
It is preferred that the capacitor is set to satisfy a relationship of Vth less than {(Cm+Cgd)/(Cm+Cgd+Cgs)}Vdd, wherein Vth is a threshold voltage of the MOS transistor, Cm is a capacitance of the capacitor, Cgs is a capacitance between the gate and source of the MOS transistor whose drain is subject to application of a power supply voltage, Cgd is a capacitance between the gate and drain of the MOS transistor whose drain is subject to application of a power supply voltage, and Vdd is a voltage of a power supply. In particular, it is preferred that the capacitor is set to satisfy a relationship of Vth less than 0.8{(Cm+Cgd)/(Cm+Cgd+Cgs)}Vdd.
It is also preferred that the resistor is set to satisfy a relationship of 1xc3x9710xe2x88x927 less than Rm(Cm+Cgd+Cgs) less than 5xc3x9710xe2x88x926, wherein Rm is a resistance of the resistor, Cm is a capacitance of the capacitor, Cgs is a capacitance between the gate and source of the MOS transistor whose drain is subject to application of a power supply voltage, and Cgd is a capacitance between the gate and drain of the MOS transistor whose drain is subject to application of a power supply voltage.
The above various relationships are useful in optimizing the circuit.
As a second aspect of the present invention, there is provided a flywheel diode comprises a first conductive type of semiconductor substrate; a second conductive type of flywheel-diode-forming impurity diffusion region formed in a surface layer part of a first surface; an anode electrode formed on the first surface so as to contact the flywheel-diode-forming impurity diffusion region; and a cathode electrode formed on a second surface of the semiconductor substrate, wherein a first conductive type of region is exposed from the first surface of the semiconductor substrate, a first conductive type of MOS-transistor impurity diffusion region is partly formed in a surface layer part of the flywheel-diode-forming impurity diffusion region, a gate electrode is formed above between the exposed first conductive type of region and the MOS-transistor impurity diffusion region via a gate insulating layer, and the anode electrode is brought into contact with both of the flywheel-diode-forming impurity diffusion region and the MOS-transistor impurity diffusion region. This structure makes it easier to embed a MOS transistor into a chip in which the flywheel diode is formed.
As a third aspect of the present invention, there is a load drive circuit for driving an inductive load, in which two or more pairs of two power MOS transistors mutually connected in series are connected pair by pair to positive and negative power lines, each MOS transistor including a body diode serving as a flywheel diode, and a line mutually connecting the paired two power MOS transistors are connected to the inductive load, wherein the paired two power MOS transistors are operated by turns by controlling a gate voltage of each power MOS transistor. The load drive circuit comprises a capacitor of which both ends are connected to a gate and a drain of each power MOS transistor; a resistor of which both ends are connected to the gate and a source of each power MOS transistor; and means for controlling a gate-to-source voltage of each power MOS transistor by causing both the capacitor and resistor to raise the gate-to-source voltage up to a value over a threshold voltage given the power MOS transistors for a predetermined period of time during a recovery operation of the flywheel diode.
Accordingly, in the same manner as above, the recovery characteristic of the flywheel diode can be softened (or smoothed), thus recovery surges being suppressed.
This load drive circuit can be embodied into various forms. For example, an external flywheel diode can be connected to the MOS transistor, not being embedded within the MOS transistor. The MOS transistor can be replaced by an IGBT element.
As a fourth aspect of the present invention, a chip into which each power MOS transistor is incorporated is mounted on a metal plate. The drain of the power MOS transistor is electrically connected to the metal plate through the chip, and both of the metal plate and a gate-lead frame are connected to a chip capacitor composing the capacitor. This provides the circuit with a practical structure.